This invention is directed to exposure control for an x-ray or similar radiographic system. The invention is more particularly concerned with a circuit that is used in conjunction with a phototimer or other automatic control element for an x-ray system to capture the x-ray generator exposure time and replicate that time on subsequent exposures.
In many areas of radiography where a series of exposures is needed, a phototimer or similar device is often used to control the exposure time of the x-ray generator. The time of exposure is based on the accumulated dose received in the image producing element. The phototimer acts to terminate the radiation from the x-ray generator typically after about 1 msec to 150 msec. The absorptivness or transparency of the subject's body tissues are factors that can affect the length of the exposure time. That is, for a particular dose setting, a thicker body part will have a longer exposure time than a thinner body part, and bone has a longer exposure time than soft tissues.
Phototimed exposure is employed in digital subtractive angiography (DSA) where a sequence of exposures are acquired serially. Typical acquisition rates can be one to two frames per second, with the sequence containing from ten to forty exposures.
Higher quality images can be acquired if the density of each image is kept constant over an entire sequence. Variation of exposure time from frame to frame, which occurs in phototimed DSA, can lead to reduced image quality.
In DSA, an opacifying agent or contrast agent (typically a compound of iodine) which is a strong absorber of X-rays is injected into the patient, and the sequence of X-ray images is acquired as the contrast agent courses through the subject's blood vessels. The contrast agent attentuates X-rays to a larger extent than the surrounding tissues, and permits the blood vessels to be distinguished. In DSA, the first few images are acquired without contrast agent in the vessel. Then the amount of contrast agent builds up, making the vessel more and more opaque. After maximum opacity, the amount of contrast agent subsides. The first few images, where there is no or little contrast agent present, serve as mask images, and can be subtracted from subsequent images to subtract out any tissues other than the vessels, i.e., bone and muscle. As the contrast agent enters the vessel, it causes the vessel to absorb more and more radiation making the vessel more opaque, but also lengthening the exposure time in the case of phototimed exposures. As a result, the length of the exposure is different from one image to another. Because exposure time is different, image density is different in the non-contrast areas (i.e., surrounding tissues). Thus when the images are subtracted from one another, the resulting image is of reduced quality.
Phototimed exposures may also yield inconsistent exposure times due to inherent tolerances and from the fact that they respond in direct proportion to the amount of light that reaches them.
In the case that a phototimer or similar device is not used, a fixed exposure technique can be used, where the technologist sets a predetermined fixed time on the console. With the fixed exposure technique the technologist selects current (ma) and voltage (KV) for the x-ray tube, based mostly on experience, and then shoots a scout image. The scout image is then evaluated in terms of its brightness and contrast. If the image is lacking these in areas, the current and voltage are adjusted and another scout images is taken. When the current and voltage are optimized and the scout image indicates good contrast, brightness, and density, the DSA examination can proceed.
The fixed exposure methodology is cumbersome because one or usually more than one scout images are required. This consumes time, and exposes the patients to additional radiation they would not otherwise receive.
Thus, between the two conventional techniques, a phototimer methodology will generally lead to poorer quality images than can be gained using a fixed exposure time technique, but the fixed time methodology is cumbersome and exposes the patient unnecessarily to additional radiation.